Exhaust gas control apparatus for internal combustion engine

ABSTRACT

A converter ( 70 ) which houses a NOx storage/reduction catalyst is provided in an exhaust passage ( 2 ) of an engine ( 1 ). At a front half portion (inlet side portion) ( 7   a ) of a carrier of the NOx storage/reduction catalyst in the converter ( 70 ), the amount of an oxygen storage component is made less than it is at a rear half portion (outlet side portion) ( 7   b ) of the carrier, and a NOx storage capacity is made larger than it is at the rear half portion ( 7   b ) of the carrier. As a result, unpurified NOx released from the front half portion ( 7   a ) of the carrier at the beginning of a rich spike due to an 02  storage operation is able to be stored in the rear half portion ( 7   b ) of the carrier, and so is not exhausted outside the catalyst Moreover, the amount of HC and CO components in the exhaust gas that are needlessly consumed by the 02  storage operation without being used to purify NOx is reduced, making it possible to purify NOx efficiently.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an exhaust gas control apparatus for aninternal combustion engine, and more specifically to an exhaust gascontrol apparatus for an internal combustion engine provided with a NOxstorage/reduction catalyst.

2. Description of the Related Art

A NOx storage/reduction catalyst is known which stores NOx in theexhaust gas by at least one of adsorption and absorption when theair-fuel ratio of in-flowing exhaust gas is lean, and then reduces andpurifies the stored NOx using, for example, HC and reduction componentssuch as CO and H₂ (hereinafter these will be collectively termed“reduction components”) in the exhaust gas when the air-fuel ratio ofthe in-flowing exhaust gas is rich.

One such exhaust gas control apparatus for an internal combustion enginethat uses this type of NOx storage/reduction catalyst is disclosed inJapanese patent laid open application No. JP(A) 2000-154713.

The apparatus disclosed in this publication improves the NOxpurification efficiency of the NOx storage/reduction catalyst bycarrying an oxygen storage component on only the front half of a carrierof the NOx storage/reduction catalyst.

When a three-way catalyst is provided in an exhaust passage on theupstream side of the NOx storage/reduction catalyst and that three-waycatalyst has an oxygen (O₂) storage function, the exhaust gas controlperformance of the NOx storage/reduction catalyst may decline due to adelay in the change in the air-fuel ratio of the exhaust gas flowinginto the NOx storage/reduction catalyst.

As is well known, a three-way catalyst can be made to have an oxygenstorage function by carrying on it metal components such as cerium Ce asan auxiliary agent, in addition to a precious metal catalyst componentsuch as platinum Pt, palladium Pd, or rhodium Rh. That is, ceriumcarried on a catalyst as an additive agent stores oxygen by bonding tothe oxygen in the exhaust gas when the air-fuel ratio of the exhaust gasflowing into the catalyst is higher than the stoichiometric air-fuelratio (i.e., when the air-fuel ratio of the exhaust gas is lean) toproduce ceria (cerium oxide IV: CeO₂). Further, when the air-fuel ratioof the in-flowing exhaust gas is equal to, or less than, thestoichiometric air-fuel ratio (i.e., the air-fuel ratio of the exhaustgas is rich), the ceria releases the oxygen to produce cerium oxide III(Ce₂O₃).

Therefore, in three-way catalyst having an O₂ storage function, oxygenis released from the three-way catalyst when the air-fuel ratio of theexhaust gas changes from lean to rich. Even if the air-fuel ratio of theexhaust gas flowing into the three-way catalyst changes to rich, theair-fuel ratio of the exhaust gas passing through the three-way catalystis maintained near the stoichiometric air-fuel ratio while oxygen isbeing released from the three-way catalyst.

When the three-way catalyst provided in an exhaust passage on theupstream side of the NOx storage/reduction catalyst has an O₂ storagefunction, even if the air-fuel ratio of the exhaust gas from the enginechanges from lean to rich during a rich spike operation of the engine,the air-fuel ratio of the exhaust gas flowing into the NOxstorage/reduction catalyst does not immediately become rich, but ratheris temporarily maintained near the stoichiometric air-fuel ratio. Thatis, the reduction components in exhaust gas having a rich air-fuel ratioare oxidized by the oxygen released from the oxygen storage component ofthe catalyst, such that exhaust gas with an air-fuel ratio near thestoichiometric air-fuel ratio and which contains relatively fewreduction components flows into the NOx storage/reduction catalyst.

Meanwhile, NOx is released from the NOx storage/reduction catalyst whenthe air-fuel ratio of the exhaust gas changes (drops) from a leanair-fuel ratio to an air-fuel ratio near the stoichiometric air-fuelratio, but the exhaust gas that flows into the NOx storage/reductioncatalyst contains only a relatively small amount of reductioncomponents, not enough to reduce the entire amount of NOx that isreleased. As a result, the NOx that was released from the NOxstorage/reduction catalyst and not reduced may flow out from thedownstream side of the NOx storage/reduction catalyst.

Because of this, the apparatus disclosed in Japanese laid openapplication No. JP(A) 2000-154713 improves the NOx purificationefficiency of the NOx storage/reduction catalyst by applying the O₂storage function to the front half portion of the NOx storage/reductioncatalyst or providing a three-way catalyst having an O₂ storage functionadjacent to, and on the upstream side of, the NOx storage/reductioncatalyst.

Accordingly, by providing the three-way catalyst having an O₂ storagefunction on the upstream side of the NOx storage/reduction catalyst inthis apparatus, the reduction components in the exhaust gas are oxidizedby oxygen released from the ceria when the air-fuel ratio of thein-flowing exhaust gas is rich. Reaction heat from that reaction raisesthe temperature of the NOx storage/reduction catalyst component carriedon the carrier, which promotes the release of NOx from the NOxstorage/reduction catalyst and improves the catalyst activity. This isbelieved to increase the purification efficiency of the released NOx.

As described above, the apparatus disclosed in Japanese patent laid openapplication No. JP(A) 2000-154713 improves the purification efficiencyof the catalyst by carrying the oxygen storage component on only thefront half of the carrier of the NOx storage/reduction catalyst.

However, while having the oxygen storage component carried on only thefront half of the catalyst carrier is beneficial for improving catalystactivity by raising the temperature of the catalyst by oxidizing the HCand CO in the exhaust gas when the air-fuel ratio is rich, as describedin Japanese patent laid open application No. JP(A) 2000-154713, problemsstill remain. That is, during the initial period of a change from a leanair-fuel ratio to a rich air-fuel ratio, the reduction components in theexhaust gas end up becoming oxidized by the oxygen released from theoxygen storage component, which results in a shortage of reductioncomponents for reducing and purifying the NOx stored in the NOxstorage/reduction catalyst.

On the other hand, in order to solve this problem, it is also possiblenot to have the oxygen storage component be carried on (either the fronthalf or the back half) of the NOx storage/reduction catalyst carrier, asis done in the related art. However, the NOx storage/reduction catalystfunctions as a three-way catalyst that simultaneously purifies threecomponents (HC, CO, NOx) in the exhaust gas in a narrow air-fuel ratiorange near the stoichiometric air-fuel ratio. Therefore, when the engineis operated near the stoichiometric air-fuel ratio, in order toeliminate relatively small air-fuel ratio fluctuations near thestoichiometric air-fuel ratio of the exhaust gas and effectively utilizethe three-way catalyst function of the NOx storage/reduction catalyst,it is necessary to carry at least a certain amount of the oxygen storagecomponent on the carrier of the NOx storage/reduction catalyst.

That is, even though carrying the oxygen storage component on the NOxstorage/reduction catalyst carrier is problematic from the viewpoint ofNOx control, in reality it is necessary to carry the oxygen storagecomponent in order to make use of the three-way catalyst function.

SUMMARY OF THE INVENTION

It is an object of this invention to provide an exhaust gas controlapparatus for an internal combustion engine able to dramatically improveNOx purification efficiency of a NOx storage/reduction catalyst whereinthe NOx storage/reduction catalyst carries on a NOx storage/reductioncatalyst carrier, an amount of the oxygen storage component sufficientfor the NOx storage/reduction catalyst to function as a three-waycatalyst while minimizing the effects from the oxygen storage component.

First aspect of the invention relates to an exhaust gas controlapparatus for an internal combustion engine provided with a NOxstorage/reduction catalyst provided in an exhaust passage and whichstores NOx in exhaust gas by at least one of adsorption and absorptionwhen an air-fuel ratio of in-flowing exhaust gas is lean, and thenreduces and purifies the stored NOx using reduction components in theexhaust gas when the air-fuel ratio of the in-flowing exhaust gas isrich includes an upstream side portion of a carrier of the NOxstorage/reduction catalyst, which is positioned on an upstream side ofan exhaust gas flow, and a downstream side portion of the carrier of theNOx storage/reduction catalyst, which is positioned on the downstreamside of the exhaust gas flow. The carrier carries an oxygen storagecomponent that absorbs oxygen in the exhaust gas when the air-fuel ratioof the exhaust gas is lean and releases the absorbed oxygen when theair-fuel ratio of the exhaust gas is rich. Also, the amount of theoxygen storage component on the upstream side portion of the carrier ismade less than the amount of the oxygen storage component on thedownstream side portion of the carrier.

That is, both the upstream side portion and the downstream side portioncarry an oxygen storage component in addition to the NOxstorage/reduction catalyst carrier.

According to the invention, the oxygen storage component is carried onthe upstream side portion of the carrier of the NOx storage/reductioncatalyst. As a result, there is a possibility that the reductioncomponents (such as HC, CO, and H₂) in the exhaust gas may be oxidizedby oxygen that is released from the oxygen storage component when theair-fuel ratio of the in-flowing exhaust gas changes from lean to rich,and NOx that is stored in the NOx storage/reduction catalyst at theupstream side portion of the carrier may be released without first beingpurified.

However, because the amount of oxygen storage component carried on theupstream side portion of the carrier is set to a relatively smallamount, the oxygen finishes being released from the upstream sideportion of the carrier after a short period of time. Thus, after only asmall amount of the reduction components has been oxidized (i.e.,consumed), no more of the reduction components will be oxidized.Therefore, exhaust gas having a rich air-fuel ratio that includes asufficient amount of reduction components reaches the upstream sideportion of the carrier shortly after the air-fuel ratio is switched,such that all of the released NOx is reduced and purified, therebypreventing unpurified NOx from being released from the upstream sideportion of the carrier.

Also, NOx in the exhaust gas usually starts being stored from theupstream side end portion of the NOx storage/reduction catalyst.Therefore, even when the amount of NOx stored on the upstream sideportion of the carrier increases, the amount of NOx stored on thedownstream side portion of the carrier is relatively low, so there isusually sufficient room with the NOx storage capacity of the downstreamside portion of the carrier.

Furthermore, because the amount of the oxygen storage component carriedon the downstream side portion of the carrier is set to be a relativelylarge amount, a sufficient amount of oxygen is stored even after theoxygen storage component on the upstream side portion of the carrier hasreleased its absorbed oxygen.

Therefore, even when unpurified NOx is released from the upstream sideportion of the carrier when the air-fuel ratio starts to change fromlean to rich, NOx in the exhaust gas can still be stored without theair-fuel ratio at the downstream side portion of the carrier droppingvery much. As a result, unpurified NOx released from the upstream sideportion of the carrier when the air-fuel ratio starts to change isstored again in the downstream side portion of the carrier and thereforedoes not flow out from the downstream side of the NOx storage/reductioncatalyst.

Also, while the amount of the oxygen storage component stored on theupstream side portion of the carrier is set to be small, the amount ofthe oxygen storage component stored on the downstream side portion ofthe carrier is set to be large. As a result, it is possible for asufficient amount of the oxygen storage component to be carried on theentire NOx storage/reduction catalyst. Accordingly, the NOxstorage/reduction catalyst is able to display sufficient performance asa three-way catalyst even during operation near the stoichiometricair-fuel ratio.

According to the first aspect of the invention, it is possible todramatically improve the NOx purification efficiency of the NOxstorage/reduction catalyst by minimizing the adverse effects of theoxygen storage component on the NOx purification and while carrying, onthe NOx storage/reduction catalyst carrier, an amount of the oxygenstorage component sufficient to make the NOx storage/reduction catalystfunction as a three-way catalyst.

In the first aspect of the invention, the NOx storage capacity of theupstream side portion of the carrier may be made greater than the NOxstorage capacity of the downstream side portion of the carrier. That is,the NOx storage capacity of the upstream side portion of the carrier maybe set relatively large. As a result, a relatively large amount of NOxcan be stored, reduced, and purified mainly at the upstream side portionof the carrier, which carries a small amount of the oxygen storagecomponent of the NOx storage/reduction catalyst. Accordingly, it ispossible to efficiently reduce and purify the stored NOx with only asmall amount of reducing agents and the like consumed by the oxygenstorage component at the start (at the beginning of the change from alean air-fuel ratio to a rich air-fuel ratio) of reduction/purification.

Furthermore, the upstream side portion of the carrier and the downstreamside portion of the carrier may be carried at least one of platinum,palladium and rhodium, and the NOx storage capacity of the upstream sideportion of the carrier may be made greater than the NOx storage capacityof the downstream side portion of the carrier by changing an amount ofat least one of platinum, palladium and rhodium carried on the upstreamside de portion of the carrier and the downstream side portion of thecarrier. That is, the NOx storage capacity of the upstream side portionof the carrier of the NOx storage/reduction catalyst and the downstreamside portion of the carrier of the NOx storage/reduction catalyst can bechanged by changing the amount of at least one of platinum, palladium,and rhodium component carried on each of those portions. Most of the NOxcomponents in the exhaust gas of the internal combustion engine arenitrogen monoxide (NO). However, because the NOx storage/reductioncatalyst can only store oxides of nitrogen (NOx) in the form of NO₂, itfirst must oxidize the NO to convert it to NO₂. Further, because atleast one of platinum, palladium, and rhodium functions as an oxidationcatalyst in an oxidized atmosphere (i.e., when the air-fuel ratio islean), NO within the exhaust gas is able to be oxidized to produce NO₂.

Accordingly, by changing the amount of at least one of platinum,palladium, and rhodium component carried on the upstream side anddownstream side portions of the carrier of the NOx storage/reductioncatalyst so as to make the amount of at least one of platinum,palladium, and rhodium on the upstream side portion of the carrierlarge, for example, the amount of NO₂ produced at the upstream sideportion of the carrier will increase so that more NO₂ is able to bestored per unit volume of the carrier. That is, the NOx storage capacitycan be changed by changing the amount of at least one of platinum,palladium, and rhodium carried on the carrier.

Further, the NOx storage capacity of the upstream side portion of thecarrier may be made greater than the NOx storage capacity of thedownstream side portion of the carrier by changing at least one of acarrier cell shape, a carrier cell size, and a carrier cell number onthe upstream side portion of the carrier and the downstream side portionof the carrier. This enables the NOx storage capacity of the upstreamside portion of the carrier to be larger than the NOx storage capacityof the downstream side portion of the carrier. That is, the NOx storagecapacities of the upstream side portion of the carrier and thedownstream side portion of the carrier can be changed by changing atleast one of the cell shape, cell size, and cell number of the carrier.

For example, the cell number per unit volume of the carrier can bechanged by changing the cell shape (the shape of the cross section ofthe exhaust gas flow path) of the carrier or by changing the cell size.However, changing the cell number (i.e., the hole ratio) per unit volumeenables the amount of catalyst component carried per unit volume of thecarrier (i.e., the amount of coating material) to be increased ordecreased and still have the same coating thickness on each cellsurface.

Accordingly, it is possible to increase the amount of NOxstorage/reduction catalyst carried on the upstream side, and thereforeincrease the NOx storage capacity by, for example, making the shape ofthe cells of the downstream side portion of the carrier a normalquadrangle and making the shape of the cells of the upstream sideportion of the carrier a polygon such as a hexagon, or by increasing thecell density by making the diameter of the cells on the upstream sideportion of the carrier smaller than the diameter of the cells on thedownstream side portion of the carrier.

In this case as well, the oxygen storage capacity of the upstream sideportion of the carrier can be made smaller than the oxygen storagecapacity of the downstream side portion of the carrier by, for example,making the amount of oxygen storage component (such as ceria) carried onthe upstream side portion of the carrier less than the amount carried onthe downstream side portion.

In the first aspect of the invention, the upstream side portion of thecarrier and the downstream side portion of the carrier may be providedseparately. This is convenient for changing the cell shape or cell sizeof the carrier when changing the NOx storage capacity.

In the first aspect of the invention, the upstream side portion of thecarrier and the downstream side portion of the carrier may be providedintegrally.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows the construction of an exemplary embodimentin which the invention is applied to an internal combustion engine of anautomobile;

FIG. 2 is a view illustrating one example of a construction of aconverter shown in FIG. 1;

FIG. 3 is a view illustrating the NOx purification efficiency of a NOxstorage/reduction catalyst according to this exemplary embodiment;

FIG. 4 is a view illustrating the change over time in NOx concentrationin exhaust gas at the outlet of the NOx storage/reduction catalystaccording to the exemplary embodiment;

FIG. 5 is a view of another example of a construction of a converterthat is different from the example shown in FIG. 1; and

FIG. 6 is a view of yet another example of a construction of a converterthat is different from the examples shown in FIG. 1 and FIG. 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following description and the accompanying drawings, the presentinvention will be described in more detail in terms of exemplaryembodiments.

FIG. 1 schematically shows the construction of an exemplary embodimentin which the invention is applied to an automobile engine.

In the drawing, an internal combustion engine for an automobile(hereinafter simply referred to as “engine”) 1 is a four cylindergasoline engine having four cylinders #1 to #4. Each cylinder #1 to #4is provided with a fuel injection valve 111 to 114 which injects fueldirectly into the cylinder. As will be described later, the engine 1 inthis exemplary embodiment is a lean burn engine capable of operatingwith an air-fuel ratio higher than the stoichiometric air-fuel ratio(i.e., a lean air-fuel ratio).

Also in this exemplary embodiment, the cylinders #1 to #4 are dividedinto two cylinder groups, each group including two cylinders which donot have sequential ignition timings (i.e., two cylinders that do notfire sequentially). For example, in the exemplary embodiment shown inFIG. 1, the cylinder firing order is 1-3-4-2, so one group includescylinders #1 and #4 and the other group includes cylinder #2 and #3.Also, an exhaust port of each cylinder in any one group is connected toan exhaust manifold for that cylinder group, which is in turn connectedto an exhaust passage for that cylinder group.

In FIG. 1, reference numeral 21 a denotes an exhaust manifold in whichthe exhaust ports of the cylinder group that includes cylinders #1 and#4 are connected to an individual exhaust passage 2 a. Likewise,reference numeral 21 b denotes an exhaust manifold in which the exhaustports of the cylinder group that includes cylinders #2 and #3 areconnected to an individual exhaust passage 2 b. In this exemplaryembodiment, three-way catalysts which serve as start catalysts(hereinafter simply referred to as “SCs”) 5 a and 5 b are arranged inthe individual exhaust passages 2 a and 2 b, respectively. Also, theseindividual exhaust passages 2 a and 2 b join into a common exhaustpassage 2 on the downstream side of the SCs.

A converter 70, in which a NOx storage/reduction catalyst 7, to bedescribed later, is housed in a casing 70 a, is provided in the commonexhaust passage 2. The construction of this converter 70 will bedescribed later.

As shown in FIG. 1, upstream side air-fuel ratio sensors 29 a and 29 bare provided on the upstream side of the SCs 5 a and 5 b in theindividual exhaust passages 2 a and 2 b. A downstream side air-fuelratio sensor 31 is provided on the downstream side of the converter 70in the exhaust passage 2. These air-fuel ratio sensors 29 a, 29 b, and31 are so-called linear air-fuel ratio sensors that output voltagesignals corresponding to the exhaust gas air-fuel ratio over a wideair-fuel ratio range.

An electronic control unit (ECU) 30, which in this exemplary embodimentis a microcomputer of a well known construction including RAM, ROM and aCPU, performs basic controls such as ignition timing control and fuelinjection control of the engine 1. Also in this exemplary embodiment, inaddition to performing the aforementioned basic controls, the ECU 30also performs a control to change the operating air-fuel ratio of theengine by changing the fuel injection mode of the in-cylinder injectionvalves 111 to 114 according to the engine operating state, as will bedescribed later.

Various signals are input into an input port of the ECU 30. For example,signals indicative of the exhaust gas air-fuel ratios on the upstreamsides of the SCs 5 a and 5 b are input from the upstream side air-fuelratio sensors 29 a and 29 b; a signal indicative of the exhaust gasair-fuel ratio on the downstream side of the converter 70 is input fromthe air-fuel ratio sensor 31; a signal corresponding to an intake airpressure of the engine is input from an intake air pressure sensor 33provided in the engine intake manifold, not shown; and a signalcorresponding to the engine speed is input from an engine speed sensor35 provided near the engine crankshaft (not shown).

Further, according to this exemplary embodiment, a signal indicative ofan accelerator depression amount (accelerator opening amount) by adriver is input to the input port of the ECU 30 from an acceleratoropening amount sensor 37 positioned near an accelerator pedal, notshown, of the engine 1. Also, an output port of the ECU 30 is connectedto the fuel injection valves 111 to 114 of the cylinders via fuelinjection circuits, not shown, in order to control the fuel injectionquantity and fuel injection timing for each of the cylinders.

Next, the converter 70 according to the exemplary embodiment will bedescribed.

FIG. 2 is a sectional view showing the construction of the converter 70according to the exemplary embodiment. The converter 70 is formed withthe NOx storage/reduction catalyst 7 housed in the casing 70 a.

The NOx storage/reduction catalyst 7 of the exemplary embodiment uses acarrier of cordierite, for example, having a honeycomb construction, forexample, with an alumina coating on its surface. The alumina layercarries a precious metal such as platinum Pt, palladium Pd, and rhodiumRh and at least one component selected from among an alkali metal suchas kalium K, natrium Na, lithium Li, and cesium Cs, an alkaline earthmetal such as barium Ba and calcium Ca, and a rare-earth metal such aslanthanum La, cerium Ce and yttrium Y. The NOx storage/reductioncatalyst performs a NOx absorption/release operation in which it absorbsNOx (NO₂ and NO) in the exhaust gas in the form of nitrate ions NO₃ ⁻when the air-fuel ratio of the in-flowing exhaust gas is lean, andreleases the absorbed NOx when the oxygen concentration in the exhaustgas drops.

For example, when the engine I is operating at a lean air-fuel ratiosuch that the exhaust gas flowing into the NOx storage/reductioncatalyst 7 has a lean air-fuel ratio, the NOx (NO) in the exhaust gasbecomes oxidized on, for example, platinum Pt so as to produce NO₂,which further oxidizes thus producing nitrate ions. When barium oxideBaO is used as an absorption agent, for example, these nitrate ionsbecome absorbed in the absorption agent and diffuse in the absorptionagent in the form of nitrate ions NO₃ ⁻ while bonding with the bariumoxide BaO. Therefore, in a lean atmosphere, NOx in the exhaust gas willbe stored in the form of nitrates.

When the oxygen concentration in the in-flowing exhaust gas decreasesdrastically (i.e., when the air-fuel ratio of the exhaust gas becomesthe stoichiometric air-fuel ratio or a rich air-fuel ratio), the amountof nitrate ions produced on the platinum Pt decreases, resulting in thereaction progressing in the opposite direction, with the nitrate ionsNO₃ ⁻ in the absorption agent being released therefrom in the form ofNO₂. In this case, when there are components in the exhaust gas thatfunction as reducing agents, such as CO, HC, and H₂, the NO₂ is reducedby these components on the platinum Pt.

Also in this exemplary embodiment, in addition to the NOxstorage/reduction catalyst component, a metal component such as ceriumCe as an oxygen storage component is also carried on the alumina layerof the carrier, thereby giving the carrier an oxygen (O₂) storagefunction.

The cerium carried on the alumina layer stores oxygen by bonding to theoxygen in the exhaust gas when the air-fuel ratio of the exhaust gasflowing into the catalyst is higher than the stoichiometric air-fuelratio (i.e., when the air-fuel ratio of the exhaust gas is lean) toproduce ceria (cerium oxide IV: CeO₂). Further, when the air-fuel ratioof the in-flowing exhaust gas is equal to, or less than, thestoichiometric air-fuel ratio (i.e., the air-fuel ratio of the exhaustgas is rich), the ceria releases the oxygen to produce cerium oxide III(Ce₂O₃).

That is, the oxygen storage component performs an O₂ storage operationin which it absorbs oxygen in the exhaust gas when the air-fuel ratio ofthe in-flowing exhaust gas is lean, and releases the absorbed oxygenwhen the air-fuel ratio of the in-flowing exhaust gas becomes rich.

Therefore, when the air-fuel ratio of the exhaust gas changes from leanto rich, for example, the oxygen storage component releases oxygen, suchthat even after the air-fuel ratio of the exhaust gas flowing into thecatalyst changes to a rich air-fuel ratio, the atmosphere in thecatalyst is maintained near the stoichiometric air-fuel ratio withoutbecoming rich while oxygen is being released.

Further, when the air-fuel ratio of the exhaust gas changes the otherway, i.e., from rich to lean, the oxygen storage component absorbsoxygen in the exhaust gas, such that even after the air-fuel ratio ofthe exhaust gas flowing into the catalyst changes to lean, theatmosphere inside the catalyst is maintained near the stoichiometricair-fuel ratio until the oxygen storage component becomes saturated withoxygen.

In this exemplary embodiment, when the amount of NOx absorbed by the NOxstorage/reduction catalyst during lean air-fuel ratio operation of theengine 1 increases, a rich spike operation is performed in which theair-fuel ratio is switched for a short time from lean to rich, such thatNOx is released from the NOx storage/reduction catalyst and is reducedand purified.

When the oxygen storage component is carried together with the NOxstorage/reduction catalyst, even when the air-fuel ratio of the exhaustgas is switched from lean to rich during a rich spike due to an O₂storage operation, reduction components such as CO and H₂ in the exhaustgas flowing into the NOx storage/reduction catalyst end up becomingoxidized by the oxygen released from the oxygen storage component, whichresults in the atmosphere within the NOx storage/reduction catalystbeing maintained at an air-fuel ratio near the stoichiometric air-fuelratio at the beginning of the rich spike.

On the other hand, as the air-fuel ratio drops, NOx is released from theNOx storage/reduction catalyst. Because the reduction components in theexhaust gas flowing into the catalyst end up becoming oxidized by theoxygen released from the oxygen storage component, however, there arenot enough reduction components necessary to reduce and purify thereleased NOx in the catalyst, as described above. Therefore, when aconventional NOx storage/reduction catalyst is made to carry an oxygenstorage component, the NOx released from the NOx storage/reductioncatalyst at the beginning of the rich spike ends up being released fromthe downstream side of the catalyst without being reduced, and thereforeunpurified, due to the shortage of reduction components.

In order to prevent this, it is possible not to provide the oxygenstorage component in the NOx storage/reduction catalyst or on theupstream side thereof so that no oxygen would be released at thebeginning of the rich spike. However, in order to improve theperformance of the NOx storage/reduction catalyst as a three-waycatalyst, it is necessary that the NOx storage/reduction catalyst carryan oxygen storage component.

For example, in a narrow air-fuel ratio range centered around thestoichiometric air-fuel ratio, platinum Pt has the capability of athree-way catalyst, in that it can simultaneously purify threecomponents (HC, CO, and NOx) in the exhaust gas.

Also, like the engine 1 in the exemplary embodiment, in an engineoperating over a broad range from a lean air-fuel ratio to a richair-fuel ratio, there are many opportunities for the engine to operateat the stoichiometric air-fuel ratio. Therefore, in this exemplaryembodiment, it is necessary to maximize use of the capability of the NOxstorage/reduction catalyst 7 as a three-way catalyst at thestoichiometric air-fuel ratio.

As described above, in order for the NOx storage/reduction catalyst topurify the three components (i.e., HC, CO, and NOx) simultaneously, theair-fuel ratio of the exhaust gas must be in a narrow range centeredaround the stoichiometric air-fuel ratio. Therefore, even if theair-fuel ratio fluctuates somewhat while the engine 1 is operating atthe stoichiometric air-fuel ratio, the O₂ storage effect achieved by theexhaust gas storage component is necessary to maintain the atmosphereinside the NOx storage/reduction catalyst near the stoichiometricair-fuel ratio and display the three-way capability.

Therefore, conventionally, in order to effectively utilize the three-waycapability of a NOx storage/reduction catalyst, a certain amount of anoxygen storage component is carried together with the NOxstorage/reduction catalyst on the carrier. As described above, however,the oxygen storage component results, for example, in unpurified NOxcomponents being released at the beginning of a rich spike operation,such that the overall NOx purification efficiency is unable to beimproved.

This exemplary embodiment solves this problem by dividing the catalystcarrier into an upstream side portion (front half) 7 a and a downstreamside portion (back half) 7 b and changing the O₂ storage capacity andthe NOx storage capacity of each carrier portion.

That is, in this exemplary embodiment, the relationship between the O₂storage capacities and the NOx storage capacities of the upstream sideportion 7 a and the downstream side portion 7 b are set as follows.

(1) The O₂ storage capacity of the upstream side portion 7 a is lessthan the O₂ storage capacity of the downstream side portion 7 b.

(2) The NOx storage capacity of the upstream side portion 7 a is greaterthan the NOx storage capacity of the downstream side portion 7 b.

The effects of each are as follows.

(1) An effect is achieved by setting the O₂ storage capacity (OSC) ofthe upstream side portion 7 a less than the O₂ storage capacity of thedownstream side portion 7 b.

The effects achieved by making the oxygen storage capacity of theupstream side portion 7 a less than the oxygen storage capacity of thedownstream side portion 7 b are as follows.

That is, when the NOx storage/reduction catalyst absorbs NOx in theexhaust gas, the storage amount increases from the upstream side portionfirst. Therefore, even when the amount of NOx stored at the upstreamside portion reaches a certain value such that a rich spike is executed,the amount of NOx stored at the downstream side portion is usuallyrelatively small so there is still sufficient room to store NOx there.

When a rich spike is executed in this state, reduction components in theexhaust gas are consumed at the beginning of the rich spike due to theoxygen storage capacity (OSC) described above, so NOx released at theupstream side portion of the catalyst flows into the downstream sideportion without being purified.

Because the OSC of the downstream side portion is relatively large sothat a relatively large amount of oxygen is released, the air-fuel ratiothere does not drop much. Also, because the amount of NOx absorbed atthe downstream side portion is less than it is at the upstream sideportion, as described above, there is still sufficient room to store NOxthere.

Accordingly, the unpurified NOx that flows out from the upstream sideportion is stored in the NOx storage/reduction catalyst, which preventsit from flowing out from the downstream side of the catalyst.

On the other hand, because the OSC of the upstream side portion issmall, the reduction components in the exhaust gas are only consumed bythe released oxygen for a short period of time, such that a sufficientamount of reduction components can be supplied inside the NOxstorage/reduction catalyst thereafter. As a result, the NOx is releasedand reduced and purified in sequence from the upstream side portion ofthe NOx storage/reduction catalyst.

That is, the exemplary embodiment achieves the following: a) by makingthe OSC of the downstream side portion of the NOx storage/reductioncatalyst large, unpurified NOx released from the upstream side portionof the NOx storage/reduction catalyst at the beginning of a rich spikeis re-stored at the downstream side portion and thus prevented fromflowing out at the downstream side, and b) by making the OSC of theupstream side portion of the NOx storage/reduction catalyst small, theamount of oxygen released at the beginning of the rich spike is reducedso the amount of unpurified NOx released from the upstream side portionis less. Moreover, the amount of reduction components in the exhaust gasneedlessly consumed that are not used to purify NOx is reduced, makingit possible to purify NOx efficiently.

When all of the NOx stored in the upstream side portion is reduced andpurified during a rich spike, the reduction components such as CO and HCin the exhaust gas pass through the upstream side portion and flow intothe downstream side portion. In order to prevent unpurified HC and COand the like from flowing out of the downstream side of the catalyst,there was conventionally a need to end the rich spike before all of theNOx stored in the NOx storage/reduction catalyst is reduced, i.e.,before the NOx storage amount reaches zero.

In contrast, according to this exemplary embodiment, because the OSC ofthe downstream side portion is set large, even if exhaust gas containinglarge amounts of HC and CO flows from the upstream side portion of theNOx storage/reduction catalyst into the downstream side portion thereof,that HC and CO are able to be purified by the oxygen released from thedownstream side portion so no unpurified HC or CO flows out from the NOxstorage/reduction catalyst. Therefore, according to this exemplaryembodiment, a rich spike is able to be performed until the NOx storageamount of the upstream side portion completely reaches zero withoutunpurified HC and CO and the like flowing out, thereby enabling NOx tobe purified efficiently.

Also, in this exemplary embodiment, even though the OSC of the upstreamside portion is less than it is conventionally, it is possible to makethe OSC of the downstream side greater than it is conventionally.Therefore, the OSC of the overall NOx storage/reduction catalyst can beset equal to, or greater than, what it is conventionally set to withouta decline in the purification efficiency of the NOx, such that it ispossible to effectively utilize the three-way catalyst capability of theNOx storage/reduction catalyst near the stoichiometric air-fuel ratio.

(2) An effect is achieved by making the NOx storage capacity of theupstream side portion 7 a greater than the NOx storage capacity of thedownstream side portion 7 a.

As described above, by making the NOx storage capacity of the upstreamside portion 7 a greater than the NOx storage capacity of the downstreamside portion 7 b, in addition to making the O₂ storage capacity of theupstream side portion 7 a less than the O₂ storage capacity of thedownstream side portion 7 b, the exemplary embodiment achieves thefollowing effects.

That is, as described above, with the NOx storage/reduction catalystaccording to the exemplary embodiment, the OSC of the upstream sideportion is set small so the amount of reduction components in theexhaust gas that are needlessly consumed at the beginning of a richspike is relatively small. Also, in the upstream side portion, it ispossible to completely purify the stored NOx without exhaustingunpurified HC and CO components outside the catalyst. Accordingly, it ispossible to reduce and purify stored NOx far more efficiently thannormal at the upstream side portion.

Therefore, by setting the NOx storage capacity (i.e., the amount of NOxable to be stored per unit volume of the carrier) of the upstream sideportion larger than the NOx storage capacity of the downstream sideportion, it is possible to purify primarily NOx in the exhaust gas atthe upstream side portion where NOx can be purified efficiently, andthus possible to drastically improve overall NOx purification efficiencycompared with conventional technology.

FIG. 3 is a graph showing a change in the NOx purification efficiency(i.e., the amount of NOx in the exhaust gas flowing out of the catalystdivided by the amount of NOx in the exhaust gas flowing into thecatalyst) between (A) a conventional NOx storage/reduction catalysthaving a uniform OSC and NOx storage capacity distribution over theentire catalyst carrier, (B) a case in which the OSC of the upstreamside portion of the carrier is set to be less than the OSC of thedownstream side portion (in the case of (1) above), and (C) a case inwhich, in addition to the OSC of the upstream side portion of thecarrier being set to less than the OSC of the downstream side portion,the NOx storage capacity of the upstream side portion of the carrier isset to be greater than the NOx storage capacity of the downstream sideportion (in the case of (2) above).

The vertical bars in FIG. 3 show the overall NOx purification efficiencyin each of the above cases when a rich spike is performed each time theNOx storage amount of the NOx storage/reduction catalyst reaches apredetermined amount (800 mg).

As shown in the drawing, in the case of (A) with the conventional NOxstorage/reduction catalyst having a uniform OSC and NOx storage capacitydistribution over the entire catalyst carrier, the overall NOxpurification efficiency is relatively low. Compared to this, in the caseof (B) in which the OSC of the upstream side portion of the carrier isset to be small, the overall NOx purification efficiency improves.Furthermore, in the case of (C) in which the NOx storage capacity of theupstream side portion is set large in addition to the OSC of theupstream side portion of the carrier being set small, the NOxpurification efficiency improves drastically.

In addition, FIG. 4 shows a change in the concentration of NOx in theexhaust at the outlet of the NOx storage/reduction catalyst under actualuse conditions. In the graph, the horizontal axis indicates time and thevertical axis indicates the NOx concentration at the outlet of the NOxstorage/reduction catalyst. Lines indicated by the letters “RS” indicatethe timing at which the rich spike operation is executed (a rich spikelasting for 1 second is repeated every 60 seconds in the example shownin FIG. 4).

In FIG. 4, broken line A shows the conventional NOx storage/reductioncatalyst having a uniform OSC and NOx storage capacity distribution overthe entire catalyst carrier, and solid line C shows the NOxstorage/reduction catalyst in which the OSC of the upstream side portionof the carrier is set smaller than the OSC of the downstream sideportion and the NOx storage capacity of the upstream side portion is setlarger than the NOx storage capacity of the downstream side portion (inthe case of C in FIG. 3).

With the conventional NOx storage/reduction catalyst (broken line A),when a rich spike is executed, a relatively large amount of unpurifiedNOx is released at the beginning of the rich spike due to the effect ofthe OSC (shown by al in FIG. 4). Further, when the NOx starts to bestored after the rich spike ends, the NOx concentration at the outlet ofthe NOx storage/reduction catalyst does decrease, but because the storedNOx amount does not completely become zero even when the rich spikeends, the NOx concentration at the outlet increases in a relativelyshort time such that it ends up being relatively high (a2 in FIG. 4)right before the start of the next rich spike.

In contrast, in the exemplary embodiment (solid line C), because the OSCof the upstream side portion of the carrier is set low, and amount ofunpurified NOx released at the beginning of a rich spike is small.Further, because the OSC of the downstream side portion is set large,the unpurified NOx that is released from the upstream side portion isstored again in the downstream side portion, preventing it from flowingout of the catalyst. As a result, the unpurified NOx that is released atthe beginning of a rich spike is dramatically reduced (c1 in FIG. 4).

Also in this exemplary embodiment, it is possible to effectively use thereduction components in the exhaust gas even if the rich spike timing isthe same because the OSC of the upstream side portion is set low, suchthat the amount of NOx stored in the NOx storage/reduction catalystafter the rich spike is less than it is conventionally. Furthermore, bysetting the NOx storage capacity of the upstream side portion large, NOxcan be purified efficiently using mainly that upstream side portion. Asa result, the amount of increase in the NOx concentration at the outletafter a rich spike is reduced such that NOx concentration right beforethe start of the next rich spike is dramatically low (c2 in FIG. 4).

The OSC (oxygen storage capacity) can be easily changed by changing theamount of the oxygen storage component (such as ceria) carried on thecarrier. Therefore, it is easily possible to make the OSC of theupstream side portion smaller than the OSC of the downstream side by,for example, decreasing the amount of ceria carried on the upstream sideportion of the carrier and increasing the amount of ceria carried on thedownstream side portion of the carrier. Also, the total amount of ceriacarried on the upstream side and downstream side portions is set to anamount sufficient for enabling the NOx storage/reduction catalyst 7 tofunction as a three-way catalyst near the stoichiometric air-fuel ratio.

Further, one way to increase the NOx storage capacity at the upstreamside portion of the carrier, for example, is to increase the amount ofplatinum carried on that portion. Another way is to change the cellconcentration on the upstream side portion and the downstream sideportion by changing either the number or the shape of cells on theupstream side portion and downstream side portion, if possible.

As described above, the NOx storage/reduction catalyst is capable ofstoring NOx only in the form of NO₂. Because most of the NOx in theexhaust gas is NO, the concentration of NO₂ in the exhaust gas directlyaffects the amount of NOx stored in the NOx storage/reduction catalyst.The NOx storage/reduction catalyst converts NOx into a storable form byoxidizing NO with the platinum component. Accordingly, by increasing theamount of platinum carried on the upstream side portion, the catalyst isable to convert more NO into NO₂ at that portion. As a result, more NOxis able to be stored on the upstream side portion of the NOxstorage/reduction catalyst. That is, by increasing the amount ofplatinum component carried on the upstream side portion, it possible toincrease the NOx storage capacity of that upstream side portion.

Further, the NOx storage capacity can be changed by changing not onlythe amount of platinum carried on the carrier, as described above, butalso the number of cells (i.e., the cell density) per unit volume of thecarrier. That is, increasing the cell density increases the amount ofcoating material per unit volume, even though the thickness of thewash-coat is the same. More coating material per unit volume results ina greater NOx storage capacity.

One typical method used to increase the cell density is to make the sizeof the individual cells smaller and increase the number of cells.However, it is also possible, for example, to change the cell shape fromquadrilateral in cross section to hexagonal in cross section so as toincrease the effective area per predetermined amount of coatingmaterial.

In the foregoing exemplary embodiment, a single catalyst carrier isdivided into two sections, i.e., a front half portion and a back halfportion, as shown in FIG. 2, and the OSC and NOx storage capacities ofeach are changed. The invention is not limited to this exemplaryembodiment, however. For example, the catalyst carrier may be dividedinto three or more sections and the OSC and NOx storage capacitieschanged for each section from the upstream side of the carrier towardthe downstream side of the carrier, or the OSC and the NOx storagecapacities may be changed sequentially from the upstream side endportion of the carrier to the downstream side end portion of thecarrier.

Furthermore, the carrier in the example in FIG. 2 is formed of a singleunit. However, the upstream side portion 7 a and the downstream sideportion 7 b may also be formed separately and housed in the sameconverter casing, as shown in FIG. 5, for example. Alternatively, theupstream side portion and the downstream side portion may be housed inseparate casings, as shown in FIG. 6.

In particular, when the NOx storage capacity is changed by changing theshape or size of the cells of the carrier, it is especially preferableto form the upstream side portion and the downstream side portion of thecarrier separately, as shown in FIGS. 5 and 6.

The converter 70 which houses the NOx storage/reduction catalyst isprovided in the exhaust passage 2 of the engine 1. At the front halfportion (inlet side portion) 7 a of the carrier of the NOxstorage/reduction catalyst in the converter 70, the amount of an oxygenstorage component is made less than it is at the rear half portion(outlet side portion) 7 b of the carrier, and a NOx storage capacity ismade larger than it is at the rear half portion 7 b of the carrier. As aresult, unpurified NOx released from the front half portion 7 a of thecarrier at the beginning of a rich spike due to an O₂ storage operationis able to be stored in the rear half portion 7 b of the carrier, and sois not exhausted outside the catalyst. Moreover, the amount of HC and COcomponents in the exhaust gas that are needlessly consumed by the O₂storage operation without being used to purify NOx is reduced, making itpossible to purify NOx efficiently.

1. An exhaust gas control apparatus for an internal combustion engine,provided with a NOx storage/reduction catalyst (7) provided in anexhaust passage and which stores NOx in exhaust gas by at least one ofadsorption and absorption when an air-fuel ratio of in-flowing exhaustgas is lean, and then reduces and purifies the stored NOx usingreduction components in the exhaust gas when the air-fuel ratio of thein-flowing exhaust gas is rich, the apparatus comprising: an upstreamside portion (7 a) of a carrier of the NOx storage/reduction catalyst(7), which is positioned on an upstream side of an exhaust gas flow, anda downstream side portion (7 b) of the carrier (7 a, 7 b) of the NOxstorage/reduction catalyst (7), which is positioned on the downstreamside of the exhaust gas flow, wherein the carrier (7 a, 7 b) carries anoxygen storage component that absorbs oxygen in the exhaust gas when theair-fuel ratio of the exhaust gas is lean and releases the absorbedoxygen when the air-fuel ratio of the exhaust gas is rich, and theamount of the oxygen storage component on the upstream side portion (7a) of the carrier (7 a, 7 b) is made less than the amount of the oxygenstorage component on the downstream side portion (7 b) of the carrier (7a, 7 b); characterized in that a NOx storage capacity of the upstreamside portion (7 a) of the carrier (7 a, 7 b) is made greater than theNOx storage capacity of the downstream side portion (7 b) of the carrier(7 a, 7 b).
 2. The exhaust gas control apparatus according to claim 1,characterized in that the upstream side portion (7 a) of the carrier (7a, 7 b) and the downstream side portion (7 b) of the carrier (7 a, 7 b)carry at least one of platinum, palladium and rhodium, and the NOxstorage capacity of the upstream side portion (7 a) of the carrier (7 a,7 b). is made greater than the NOx storage capacity of the downstreamside portion (7 b) of the carrier (7 a, 7 b) by changing an amount of atleast one of platinum, palladium and rhodium carried on the upstreamside portion (7 a) of the carrier (7 a, 7 b) and the downstream sideportion (7 b) of the carrier (7 a, 7 b).
 3. The exhaust gas controlapparatus according to claim 1, characterized in that the NOx storagecapacity of the upstream side portion (7 a) of the carrier (7 a, 7 b) ismade greater than the NOx storage capacity of the downstream sideportion (7 b) of the carrier (7 a, 7 b) by changing at least one of acarrier cell shape, a carrier cell size, and a carrier cell number onthe upstream side portion (7 a) of the carrier (7 a, 7 b) and thedownstream side portion (7 b) of the carrier (7 a, 7 b).
 4. The exhaustgas control apparatus according to claim 1, characterized in that theupstream side portion (7 a) of the carrier (7 a, 7 b) and the downstreamside portion (7 b) of the carrier (7 a, 7 b) are provided separately. 5.The exhaust gas control apparatus according to claim 1, characterized inthat the upstream side portion (7 a) of the carrier (7 a, 7 b) and thedownstream side portion (7 b) of the carrier (7 a, 7 b) are providedintegrally.